A/B Testing
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A popular version of this experimental design is A/B testing, where user behavior is compared between version A (the experimental group) and version B (the control group) of a site or product, which may differ in page flow, wording, imagery, colors, etc. Such experiments must be carefully designed to isolate the one factor you are studying.
From
Chapter:
Lies, Damned Lies, and Statistics
Section:
To Believe Or Not Believe